What is the correct definition of the third quartile of a data set? A. The third quartile is all the numbers of the data set added together and divided by 3. B. The third quartile is the difference between the maximum and median. C. The third quartile is the median of the upper half of data in the data set. D. The third quartile is the mode of the upper half of data in the data set.

we know that 

The third quartile (also called the upper quartile) has 75 percent of the data below it and the top 25 percent of the data above it. The third quartile is the same as the median of the part of the data which is greater than the median. 

therefore

the answer is the option
C.) The third quartile is the median of the upper half of data in the data set.